Omnidirectional ultraviolet radiation detector

ABSTRACT

An ultraviolet sensitive gaseous discharge detector has essentially uniform angular sensitivity in a plane perpendicular to the axis of the detector tube and in the plane of the detector tube axis. The cathode of the detector is in the form of a thin sputtered layer of metal adhering to the inner wall of the envelope of the detector.

United States Patent [1 1 Erickson 1 Nov. 6, 1973 OMNIDIRECTIONALULTRAVIOLET RADIATION DETECTOR [75] Inventor: Clifford W. Erickson,Minnetonka,

Minn.

[73] Assignee: Honeywell Inc., Minneapolis,

Minn.

1221 Filed: Oct. 12, 1971 211 Appl. No.: 188,398

[51] Int. Cl. H01j 39/06 [58] Field of Search 313/93, 217, 220, 313/101,218, 224; 250/83.6

[56] References Cited UNITED STATES PATENTS Panther et a1. 313/933,255,354 6/1966 Cade 313/93 X 3,022,424 2/1962 Anton 250/83.6 R2,925,509 2/1960 Hayes 3,209,197 9/1965 Ahsmann et al 313/218 X PrimaryExaminer-Palmer C. Demeo Att0rney-Lamont B. Koontz et a1.

[57] ABSTRACT An ultraviolet sensitive gaseous discharge detector hasessentially uniform angular sensitivity in a plane perpendicular to theaxis of the detector tube and in the plane of the detector tube axis.The cathode of the detector is in the fonn of a thin sputtered layer ofmetal adhering to the inner wall of the envelope of the detector.

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cathode contact means which extends through the OMNIDIRECTIONALULTRAVIOLET RADIATION I DETECTOR BACKGROUND OF THE INVENTION Thisinvention is related to radiation sensitive, gaseous discharge detectorsof the Geiger-Mueller type. More particularly, it is concerned with anultraviolet sensitive detector having omnidirectional sensitivity toultraviolet radiation.

What is meant by Geiger-Mueller detector" is a radiation detector havingan anode and a' cathode disposed in an ionizable gas, and which, uponbeing subjected to radiation to which it is sensitive, causes anelectron to be present within the electric field established by theanode and cathode, whereupon the electron accerlerates toward the anode,ionizing the gas, and causing a glow discharge current to flow, whichcurrent must be subsequently quenched by means of a quenching mechanism.

There are many applications in which an ultraviolet sensor havinguniform sensitivity over a broad viewing angle is desirable. Forinstance, fire detection systems requiring broad angle surveillanceoften utilize several sensors to achieve uniform angular sensitivity. Itis highly advantageous to utilize a single sensor rather than severalsensors.

SUMMARY OF THE INVENTION The ultraviolet radiation detector of thepresent invention exhibits omnidirectional sensitivity to ultravioletradiation. In addition, the overall sensitivity to ultraviolet radiationis approximately ten times the sensitivity of prior art ultravioletradiation detectors utilizing different electrode configurations in anenvelope of the same size. The radiation detector of the presentinvention is rugged, has a minimum of parts, and requires a minimum offabrication steps.

The envelope of the ultraviolet radiation detector of the presentinvention is transparent and has a substantially circular cross section.Contained within the envelope is an ionizable gaseous filling. A metalanode of substantially circular cross section is coaxially alignedwithin the envelope and extends through the envelope to provide externalelectrical contact. The cathode of the detector is in the form of a thinmetal layer which adheres to the inner wall of the envelope. The thinlayer of metal is sputtered from the anode during the fabrication of thedetector. The thin layer has a thickness which is less than the longerof three times the absorption length of a photon of wavelengths between2,000A. and 3,000A. in the metal and three tmes the tenuation length ofa photoelectron in-the metal. External electrical contact to the cathodeis provided by transparent envelope and makes electrical contact withthe cathode.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 4b shows an ultraviolet radiationdetector including a glass sleeve formed around the anode and extendinginto the interior of the detector.

FIG. 5 shows the angular sensitivity of the detector of FIG. 4b in aplane perpendicular to the axis of the detector.

FIG. 6 shows the angular sensitivity of the detector of FIG. 4b in theplane of the detector axis.

FIG. 7 shows a theoretical approximation of the thin cathode layer astwo parallel electrodes.

FIG. 8 shows theoretical relative sensitivity of the ultravioletradiation detector of the present invention as a function of cathodethickness for a copper cathode.

FIGS. 9a through 9c show an alternative method for providing externalelectrical contact to the cathode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 is shown a crosssectional view of an ultraviolet radiation detector of the presentinvention. Transparent envelope 10 has a substantially circular crosssection. Contained within envelope 10 is an ionizable gaseous fillingwhich may be, for example, a hydrogenhelium or a hydrogen-neon gas.mixture. Coaxially aligned within envelope 10 is metal anode 12 whichhas a substantially circular cross section. Anode 12 extends through theenvelope to provide external electrical contact. Cathode 14 is in theform of a thin layer of metal which adheres to the inner wall of theenvelope. As will be described later, the thin layer of metal is formedby sputtering material from anode 12 during one step of the fabricationof the detector. Copper, molybdenum, nickel, and tungsten are fourdesirable anode and cathode materials. In particular, copper has beenfound to be the preferred material for the anode and cathode. However,the invention is not limited to these materials.

The thin layer of metal forming cathode 14 must have a thickness whichis less than the longer of three times the absorption length of a photonof wavelengths between 2,000A. and 3,000A. in the metal and three timesthe attenuation length of a photoelectron in the metal. In the preferredembodiment the thin layer has a thickness of between the absorptionlength and the attenuation length. The sensitivity of the detector isvery dependent upon the thickness of the sputtered layer, since too thina layer does not stop enough radiation and too thick a layer does notpermit the photoelectrons to escape into the gaseous filling. When theanode and cathode are copper, the thin layer forming the cathode has athickness of less than 500A. and in its preferred embodiment has athickness of between about 50A. and about 200A.

External electrical contact to the cathode is provided by first andsecond metal pins 16a and 16b which extend into the envelope.'The end ofeach of the metal pins is flush with the inner wall of envelope 10. Thefact that pins 16a and 16b are essentially flush with the inner wall ofenvelope 10 allows them to be sputtered and make contact with the restof the thin layer forming cathode 14. Although two pins 16a and 16b areshown as forming cathode contact means it is to be understood that asingle metal pin may also be used. However, the use of two metal pinsprovides certain advantages during the fabrication of the detector, aswill be described with reference to FIG. 2. In addition, other forms ofcathode contact means may be employed. One

of these alternative means will be described with reference to FIG. 9.

FIG. 2 describes various steps in the fabrication of the detector shownin FIG. 1. In FIG. 20, glass head 20 has been formed on metal pin 16.Glass bead 20 and metal pin 16 are then ground such that one end ofmetal pin 16 is flush with theground surface of glass bead 20. A crosssectional view of the resulting structure is shown in FIG. 2b. It shouldbe noted that when two metal pins 16a and 16b are utilized as cathodecontact means, the steps described above with reference to FIGS. 2a and2b are performed twice.

First and second pins 16a and 16b and anode 12 are then aligned in thedesired spaced relationship, as shown in FIG. 20. The spacedrelationship of anodel2 and pins 16a and 16b is preferrably maintainedby clamping them in a vise or jig. A glass tube which will form envelopeis then positioned such that anode 12 and envelope 10 are coaxiallyaligned. The positioning of the glass tube is shown in FIG. 2d.

Heat is then applied to the glass tube such that first and second pin16a and 16b and anode 12 are sealed into the glass envelope 10. Thesealing of first glass bead 20a and first metal pin 16a as well assecond glass bead 20b and second metal pin 16b into the glass envelopeis such that the ground surface of the glass bead and the end of themetal pin are flush with the inner wall of glass envelope 10.

At the same time that the sealing of pin 16a and 16b and anode 12 isperformed an additional optional step may be performed. This stepinvolves application of heat to form a constriction in the glass tube.This constriction is shown in FIG. 2e and is desirable since itaccurately defines the position at which the tube will eventually besealed off.

The next step involves filling the envelope with an ionizable gaseousfilling such as a neon-hydrogen gas mixture. The gas filling apparatus30 is sealed to the end of the glass tube. The gas mixture is providedby gas supply means 32.

After the tube has been filled with the desired amount of 'ionizablegas, heat is again applied at the constriction so as to seal the glassenvelope with the gaseous filling inside. The resulting structure isshown in FIG. 2g. At this point the detector is essentially completeexcept for the thin sputtered cathode layer. To

achieve sputtering a negative voltage is applied to anode 12 withrespect to first and second pins 16a and 16b which is sufficient tocause sputtering of material from anode 12 onto the inner walls ofenvelope 10. The sputtered material covers pins 16a and 16b therebyproviding external electrical contact. The advantage of using two pins16a and 16b resides in the ability to measure the degree of ohmiccontact made by pins 16a and 16!; by measuring the resistance betweenthem. When sputtering is completed, the resulting structure is thatshown in FIG. 1.

In one successful embodiment of the present invention a copper anode wasused. The envelope was filled with a 50 per cent hydrogen and 50 percent neon gas mixture to a total pressure of 50 torr. A negative voltagewas then applied to anode 12 with respect to pins 16a and 16b. A voltageof about 400 volts was sufficient to cause the onset of abnormal glow.The current during sputtering was maintained at a constant value of lessthan five milliamps.

It is believed that the gas composition and pressure changes duringsputtering due to a preferential gettering of hydrogen. This change incomposition and pressure is indicated by a drop in breakdown voltage ofthe tube after sputtering has been performed. For this reason, it isdesirable for the fill gas composition prior to sputtering to containsomewhat more hydrogen than is required for the finished tube.

FIG. 3 shows a modified embodiment of the present invention in whichpins and 16!) are mounted in the sides of envelope 10 rather than in theend of envelope 10 as shown in FIG. 1. It was found that the detectorshown in FIG. 3 has slightly superior performance to that shown inFIG. 1. However, the detector shown in FIG. 3 is far more difficult tofabricate.

One disadvantage discovered with the detector shown in FIG. 1 is atendency to run away. A run away tube is one that is characterized byspontaneous discharges in rapid succession even after the ionizingradiation is removed. It was discovered, however, that a slightmodification of the detector of FIG. 1 alleviated this difficulty. Asshown in FIG. 4a, a glass sleeve 40 is formed around a portion of anode12 prior to the sealing of anode 12 into the glass envelope. The glasssleeve is then sealed into the glass envelope such that a portion of thesleeve extends into the interior of the detector. The finished detectoris shown in FIG. 4b. Glass sleeve 40 greatly reduces the tendency of thedetector to run away.

FIG. 5 shows the angular sensitivity of the detector of FIG. 4b in aplane perpendicular to the axis of the tube. As shown in FIG. 5, theangular sensitivity of the detector was measured by using the-naturalgas flame positioned seven feet from the detector and by having a gasflow of 138 cc per minute. The cathode of the detector was a copperlayer having a thickness of approximately 250A. to 325A. and the gasfilling prior to sputtering was a 50 per cent hydrogen and 50 per centneon gas mixture having a pressure 50 torr. The envelope had an outsidediameter of about 0.330 inches and a length of about 1 inch. As can beseen, the sensitivity of the tube is essentially identical over theentire 360. In addition, it has been found that the detector is over tentimes assensitive as detectors having the size envelope but a different,prior art electrode configuration. The increased sensitivity of thedetector of the present invention can be explained by the fact that thesensitivity of a detector is proportional to the cathode area, providedthat the incident radiation is not focused onto a particular point. Withthe present invention the area of the cathode is maximized for a givenenvelope size since the cathode consists of a thin coating on the innerwall of the envelope.

FIG. 6 shows the angular sensitivity of the detector in the plane of thetubeaxis. This plane is perpendicular to the plane of measurementdescribed with reference to FIG. 5. The sensitivity of the detector wasmeasured under conditions identical to those described with reference toFIG. 5. As can be seen from FIG. 6, the angular sensitivity of thedetector in the plane of the tube axis is also essentially uniform,thereby indicating that the detector of the present invention is trulyomnidirectional in sensitivity.

As has been described previously, the thickness of the thin layer ofmetal forming cathode 14 must be within a critical range of thicknesses.This critical range is related to the absorption length of a photon ofwavelengths between 2,000A. and 3,000A. in the metal and to theattenuation of a photoelectron in the metal. To better understand thedependence of sensitivity of the detector upon cathode thickness, it isnecessary to investigate the theory of operation of the detector of thepresent invention.

The escape probability of a photoelectron excited at a distance x fromthe surface is P (x) p e where L is the attenuation length and p is aconstant for a given photon energy. For this discussion, it is possibleto approximate the cylindrical tube by two parallel electrodes, asindicated in FIG. 7. The first electrode (the front half of the tube) isa back-illumined cathode (the electrons emerge from the side oppositethe entering photons), while the second electrode (back half of thetube) is a front-illumined cathode (electrons emerge from the side atwhich the photons enter).

The back half of the cathode is treated in a similar fashion except thatthe incident intensity is now l e since it has already passed throughthe first half, and the probability of the electron reaching the surfaceis p e since it must retrace the path of the photon. The resultingintegral is V This-process actually occurs repeatedly on each sidebecause of the reflection which occurs (about 35 percent). The aboveintegral is then multiplied by l/l-R where R is the reflectioncoefficient, to give In FIG. 8 the theoretical relative sensitivity of adetector of the present invention as a function of cathode thickness isshown when the cathode metal is copper. The value of a 9 X 10 cm 1 wasused for the theoretical calculation. This value was reported byEhrenreich and Philip in Phys. Rev. I22, I622, (1962), and is valid forwavelengths of radiation between 2,000A.

and 2,500A. The value of L, the attenuation length of photoelectron incopper, is not well known; however, a range of 50A. to 200A. has beenreported by Crowell et al., in Phys. Rev., 127, 2006, (1962). In FIG. 8the sensitivity is shown for the values of L of 50A. and 200A. It isbelieved that the value of L for copper is closer to the SOA. valuesince the attenuation length of other metals drops rapidly to this valueat energies above four electron volts.

It can be seen from FIG. 8 that when the cathode is a copper layer, thethickness is preferably less than 500A. In particular, highersensitivity is obtained when the cathode has a thickness of betweenabout 50A. and about 200A.

FIG. 9 shows an alternative method for forming the cathode contactmeans. FIG. 9a shows a cross sectional side view of the tube prior tosputtering. Two pins 50a and 50b extend'into the envelope and areconnected electrically by a springloaded fine wire 52, which is composedof two wires 52a and 52bof different sizes which are spotweldedtogether. FIG. 9b shows a top cross sectional view of the tube shown inFIG. 9a.

The sputtering process is performed with a negative voltage beingapplied to anode 12 with respect to first and second pins 50a and 50bFollowing the sputtering process, a current is passed between pins 50and 50b until the smaller wire 52b melts. The larger wire 52a is forcedby springtension against the newly sputtered surface therebyestablishing ohmic contact. FIG. 90 shows a top cross sectional view ofthe completed detector. i

It is to be understood that this invention has been disclosed withreference to a series of preferred'embodiments and it is possible tomake changes in form and detail without departing from the spirit andscope of the invention.

The embodiments of the invention in which an exclusive property or rightis claimed are defined'as follows.

1. An ultraviolet radiation detector of the photoelectron triggeredgaseous discharge type for detecting ultraviolet radiation ofwavelengths between about 2,000 A. and about 3,000 A., the ultravioletradiation detector comprising:

is less than three times the longer of the absorption length of a photonof wavelength between 2,000 A. and 3,000 A. in the metal and theattenuation length of the photoelectronin the metal, cathode contactmeans'extending through the transparent envelope and making electricalcontact with the cathode, thereby providing external electrical contactto the cathode, a metal anode coaxially aligned with the envelope andextending through the envelope to provide external electrical contact,the metal anode being spaced from the cathode, and

an ionizable gaseous filling contained in the envelope.

2. The ultraviolet radiation detector of claim I wherein the thin layerforming the cathode has a thickness of between the absorption of aphoton of wavelength between 2,000 A. and 3,000 A. in the metal and theattenuation length of a photoelectron in the metal. 3. The ultravioletradiation detector of claim 1 wherein the thin layer forming the cathodeis copper of thickness less than 500 A.

4. The ultraviolet radiation detector of claim 3 wherein the thin layerforming the cathode has a thickness of between about 50 A and about 200A.

5. The ultraviolet radiation detector of claim 1 wherein the cathodecontact means comprises a first metal pin extending into the envelope,the end of the first metal pin being flush with the inner wall of theenvelope.

metal sputtered from the metal anode.

1. An ultraviolet radiation detector of the photoelectron triggeredgaseous discharge type for detecting ultraviolet radiation ofwavelengths between about 2,000 A. and about 3,000 A., the ultravioletradiation detector compriSing: a gas tight transparent envelope, aphotoemissive cathode in the form of a thin layer of metal covering asubstantial portion of the inner wall of the envelope, the metal beingcapable of absorbing ultraviolet radiation and emitting photoelectrons,the thin layer having a thickness which is less than three times thelonger of the absorption length of a photon of wavelength between 2,000A. and 3,000 A. in the metal and the attenuation length of thephotoelectron in the metal, cathode contact means extending through thetransparent envelope and making electrical contact with the cathode,thereby providing external electrical contact to the cathode, a metalanode coaxially aligned with the envelope and extending through theenvelope to provide external electrical contact, the metal anode beingspaced from the cathode, and an ionizable gaseous filling contained inthe envelope.
 2. The ultraviolet radiation detector of claim 1 whereinthe thin layer forming the cathode has a thickness of between theabsorption of a photon of wavelength between 2,000 A. and 3,000 A. inthe metal and the attenuation length of a photoelectron in the metal. 3.The ultraviolet radiation detector of claim 1 wherein the thin layerforming the cathode is copper of thickness less than 500 A.
 4. Theultraviolet radiation detector of claim 3 wherein the thin layer formingthe cathode has a thickness of between about 50 A and about 200 A. 5.The ultraviolet radiation detector of claim 1 wherein the cathodecontact means comprises a first metal pin extending into the envelope,the end of the first metal pin being flush with the inner wall of theenvelope.
 6. The ultraviolet radiation detector of claim 5 wherein thecathode contact means further comprises a second metal pin extendinginto the envelope, the end of the second metal pin being flush with theinner wall of the envelope.
 7. The ultraviolet radiation detector ofclaim 1 wherein the ionizable gaseous filling comprises a hydrogen andneon gas mixture.
 8. The ultraviolet radiation detector of claim 1wherein the thin layer forming the cathode comprises metal sputteredfrom the metal anode.